Once WinPE boots from the Win Install ISO, it Setup will look for the 'DVD' - however, as it exists in the form of an ISO file and not a DVD device, Setup will not see it and give you the error about needing a driver as it could not find the source DVD.The grub4dos statements will cause firadisk to load the ISO as a virtual CD, but when WinPE boots it has to load firadisk as a driver first. When firadisk loads it sees the memory command find:/multiboot/ISOs/win8.x64.915440.EI.I and so loads the ISO (searches all media until it finds it and then loads it as a virtual DVD).So When each ISO boots, somehow you have to get it to run firadisk???Did you test your method fully?

Try your USB drive again and get to the bit where it asks what version to use. My guess is that it will not find the 'DVD' and ask you to install a driver so it can access the DVD drive that it is looking for.
To boot from a USB 3 port, you don't need USB 3 drivers as the BIOS will map the USB 3 drive to Int 13h BIOS calls - but as soon as you boot to an OS, then the OS will need USB 3 drivers. Win 8 already has them, Win 7 and below don't. Same for linux, most (??) linux builds don't have USB 3 drivers during the initial boot loader (initrd) stage and so will fail if you try to boot linux from a USB 3 port.

Try your USB drive again and get to the bit where it asks what version to use. My guess is that it will not find the 'DVD' and ask you to install a driver so it can access the DVD drive that it is looking for.

OK I will try that.

Same for linux, most (??) linux builds don't have USB 3 drivers during the initial boot loader (initrd) stage and so will fail if you try to boot linux from a USB 3 port.

I tried out ububtu 12.04 64-bit (is this the latest?) and that seemed to boot fine

See Tutorial #43 - your USB SSD will probably appear to Windows as a Fixed disk rather than a removable, so you will have to tweak the ISO as described in the Tutorial Addendum. For XP ISO see Tut #30.

Someone else reported the same thing. I am not sure why it happens. Mysetup.cmd should call loadiso and then it should return with the USBDRIVE variable set - for some reason it just loops???
Did you use the mysetup.cmd file from the download or the mysetup.cmd listing in the Tutorial?
I have changed the download now so that it does not test for USBDRIVE - can you try it again?